1. Field of the Invention
The present invention relates to a semiconductor device whose active region is formed from a semiconductor film constituted of a mass of crystals with various orientations (the film hereinafter is referred to as crystalline semiconductor film). Typical example of the crystalline semiconductor film is a polycrystalline silicon film. Specifically, the invention relates to a thin film transistor or a semiconductor device that has a circuit composed of the thin film transistor. The term semiconductor device herein refers to a device in general which utilizes semiconductor characteristics to function, and semiconductor integrated circuits, electro-optical devices and electronic equipment fall within this category.
2. Description of the Related Art
A technique has been developed to manufacture a thin film transistor (hereinafter referred to as TFT) from a crystalline semiconductor film with a thickness of several nm to several hundreds nm. TFTs are now established as practical switching elements for liquid crystal display devices, which has brought the recent success in forming a semiconductor integrated circuit on a glass substrate.
Silicon is a material of the crystalline semiconductor film that is suitable for a TFT. Used as this crystalline semiconductor film is a silicon film having a crystal structure (hereinafter referred to as crystalline silicon film). The crystalline silicon film is obtained by forming an amorphous silicon film on a glass or quarts substrate through deposition by plasma CVD or reduced pressure CVD and crystallizing the amorphous silicon film through heat treatment or laser light irradiation (will be called laser treatment in this specification).
When heat treatment is chosen, the amorphous silicon film has to be heated at a temperature of 600° C. or higher for 10 hours or longer to crystallize. Considering the productivity in manufacturing TFTs, it is difficult to say the method with the treatment temperature this high and the treatment time this long is a proper method. Taking a liquid crystal display device as an example of a product to which the TFTs are applied, a large-sized heat treatment furnace is required in order to accommodate the substrate as its surface are a becomes larger. This not only increases energy consumption in manufacturing process but also makes it difficult to obtain uniform crystals over the large surface area. On the other hand, when laser treatment is chosen, obtaining crystals of uniform quality is still difficult because the output of a laser oscillator is not stable. The diversity in quality between crystals results in fluctuation in characteristic between TFTs, which in turn causes lowering of display quality of the liquid crystal display device or a display device whose pixel portion is composed of light emitting elements.
Another technique has been disclosed in which a metal element for promoting crystallization of silicon is introduced in an amorphous silicon film so that a crystalline silicon film is formed by heat treatment at a temperature lower than in the conventional heat treatment. For example, Japanese Patent Application Laid-open Nos. Hei 7-130652 and Hei 8-78329 show that a crystalline silicon film can be obtained by introducing a metal element such as nickel into an amorphous silicon film and heating the film at 550° C. for four hours.
However, a TFT manufactured by using the thus formed crystalline silicon film is still inferior in characteristics to a MOS transistor comprised of a single crystal silicon substrate. If a semiconductor film with a thickness of several nm to several hundreds nm is subjected to crystallization process on a material different from the film, such as glass or quartz, only a polycrystalline structure composed of masses of plural crystal grains is obtained. In the polycrystalline structure, carriers are trapped by an infinite number of defects found in crystal grains and in grain boundaries to limit the performance of the TFT.
In the crystalline silicon film formed by the above method of prior art, crystal orientation planes are arranged at random and the orientation ratio of a specific crystal orientation is low. The crystalline silicon film obtained by heat treatment or laser treatment has plural crystal grains deposited and tends to orient in {111} orientation, although the ratio of that part that is oriented to the {111} plane to the entire film does not exceed 20%.
When the orientation ratio is low, it is nearly impossible to keep the continuity of lattice in the grain boundaries where crystals of different orientations meet, and hence many dangling bonds will presumably be generated. The dangling bonds generated in the grain boundaries work as trap centers for carriers (electrons and holes) to degrade the carrier transportation characteristic. To elaborate, carriers are scattered or trapped in such film and the crystalline semiconductor film with scattered or trapped carriers is not expected to turn into a TFT that is high in field effect mobility. Furthermore, grain boundaries are arranged at random, meaning that a channel formation region cannot be formed from crystal grains of a specific crystal orientation. This can cause fluctuation in electric characteristics of TFTs.